Combined acid and alkali process of fiber liberation



July 11, 1933. G. A. RICHTER 1,917,545

CUMBINED ACID AND ALKALI PROCESS OF FIBER LIBERATION Filed May 1, 1929 dad ml Patented July 11, 1933 UNITED STATES PATENT OFFICE GEORGE A. RICHTER, OF BERLIN, NEW HAMPSHIRE, ASSIGNOR TO BROWN COMPANY, OF BERLIN, NEW HAMPSHIRE, A CORPORATION OF MAINE COMBINED ACID AND ALKALI PROCESS OF FIBER LIBERATION Application filed Kay 1, 1929. Serial No. 359,626.

This invention relates to the production of cellulose pulp. It is concerned morepartieularly with a process in which raw cellulosiC material such as wood is treated successively with acid and alkaline liquors, the acid liquor serving to remove a portion 0 the eementitious content of the raw cellulosic material but without completing fiber liberation, and the alkaline liquor serving to remove residual cementitious material and thus to complete tiber liberation. It further relates to a process which is made up of an economical cycle of steps, the spent liquors from both acid and alkaline cooking containing the same base elcment, such as sodium, and

being simultaneously treated for the recovery of the valuable constituents present therein. By choosing various raw cellulosic materials and conditions of acid and alkali treatment, it is possible to produce final pulps of various characteristics. For instance, a final product having a pentosan content of from 4% to 8% and an alpha cellulose content of from 89% to 927 1 i and otherwise quite similar to kraft pulp may be produced by first cooking wood in an acid sulphite liquor of comparatively high combined S0 content and then cooking in an alkaline liquor to complete fiber liberation, such alkaline liquor, however, being of insufficient alkalinity to produce a fiber of high alpha cellulose content. Or a final product of high alpha cellulose content, say from 92% to 97 72,, having excellent physical characteristics and suitable for use as a raw material in the manufacture of high grade papers and in the preparation of various cellulose derivatives such as the xanthate, nitrate, acetate, and the like. may be produced by first cooking wood 40 in an acid sulphite liquor of comparatively. high free SO content and then cooking in an alkaline liquor of an alkalinity designed to producethe desired alpha cellulose content. High free SO content in the acid cooking 4 liquor is accompanied by considerable reac-,

tion upon and removal of pentosans contained in the wood, subsequent cooking in alkaline liquor effecting a removal of more non-alpha cellulose impurities and resulting in a final product of high alpha cellulose content. Even when the final product is comparable to kraft pulp, there may be advantages in a two-step cooking procedure such as described, as the initial cook may be carried out in a sulphnrous acid solution containing little, if any, sodium compounds to effect a removal of cementitious material from the wood, whereupon the subsequent alkaline cook resulting in fiber-liberation may be carried out in a sodium base alkaline liquor of lower sodium concentration than that of the usual kraft liquor, less sodium constituent, thus being handled and lost during subsequent recovery operations.

Various processes of producing pulps of high alpha cellulose content have been proposed, according towhich wood is first cooked in an acid liquor until fiber liberation has been effected, whereupon the resulting pulp is then treated at elevated temperature with alkaline liquors which serve to remove nonalpha cellulose components, including lessres5tant celluloses and ligneous matter from the. pulp. It is known that acid sulphite liquors such as a sodium bisulphite or calcium bisulphite solution are quite drastic in their action upon cellulose fiber, but such liquors have been found to be particularly effective in their action upon such non-alpha components as pentosans and ligneons material, the resulting pulp being much freer from such components than when wood is converted into fiber by cooking in liquors of an alkaline type. Because of this drastic action upon cellulose fiber, it is preferred to produce high alpha cellulose-containing pulps by cooking the wood, rather than the pulp, in such acid sulphite liquors, as the cellulose fibers while bound together are less susceptible to degradation by such liquor than after they have been isolated, the liquor tending to react selectively upon non-alpha cellulose components in the wood rather than upon the cellulose fibers.

I have observed that considerable degradation of the fiber takes place during the last stages of cooking in such acid liquors, that is, at the time when the fibers are being isolated or freed from the residual materials which cement them together. Evidently during the 10 initial stages of cooking, the acid liquor reacts with a high degree of selectivity upon the cementing material in the wood, but as this material is dissolved from the wood, the reaction becomes less selective, so that when fiber liberation is' actually taking place the liberated cellulose fibers are reacted upon and degraded appreciably by the liquor. In accordance with the process of the present invention, therefore, wood is cooked in acid liquors until a substantial portion of 1ts cementitious content is removed, but short of effecting fiber liberation, whereupon the wood is then cooked in an alkaline liquor until fiber liberation is completed. Various types of acid liquors may be used, and cooking in such liquors may be effected under awide variety of conditions, but in no case should this be continued until fiber liberation is actually efi'ected. The acid cook makes possible the removal of various non-alpha cellulose components, as, for example, a large proportion of the pentosans, as apparently the acid liquor hydrolyzes them to soluble sugars or other compounds, susceptible of solution when the wood is subsequently cooked in alkaline liquors. The acid-cooked wood when subsequently cooked in alkaline liquors may be readily converted into pulp, the alkaline liquors reacting upon and dissolving residual cementitious material and non-alpha cellulose components left undissolved by the acid liquor. Such a two-step fiber-liberating process makes possible high yields of easily bleachable pulps of high alpha cellulose content and having excellent physical characteristics, including high strength and unusually high tear resistance. Pulp of higher pentosan and lower alpha cellulose content but having excellent physical characteristics may be roduoed by using raw cellnlosic material of igh pentosan content, such as hard wood, or by choosing certain conditions of acid and alkaline cooking. The physical characteris tics of the pulp are doubtless imputable to the fact that loss of cellulose fiber due to the hydrolyzing action of the acid liquor has been reduced, the resulting pulp being of an average fiber length greater than that of the usual sulphite pulp. Inasmuch as sodium base chemicals may be used in both acids and alkaline cooking, it is possible to treat the spent liquors resulting from both cooking steps simultaneously for the recovery of the valuableconstituents presenttherein. Notonly does this result in a higher concentration of valuable inorganic constituent in the spent liquor being regenerated, but organic matter is present therein in higher proportion than is present in the usual cooking liquors, so that when the mixture of liquors is smelted in a furnace of the type in which the organic matter serves as fuel, more heat is derived therefrom. The spent alkaline liquor further serves to neutralize residual acid present in the spent acid liquor, so that a neutral mixture may be obtained which may be handled in evaporators and other apparatus which need not be acid-proof. When an acid-liquor such asan aqueous solution of sulphur dioxide, containing other acids such as sulphnric acid if desired, is used in carrying out the acid cook, the spent alkaline liquor serves to neutralize the acid content of the spent acid liquor, so that the sulphur component thereof may be recovered in the form of a sodium sulphur compound, or the acid liquor may be a sulphurous acid solution of sodium-sulphur salts, such as sodium sulphite, sodium sulphate, sodium acid sulphate, or a mixture of these salts, sodium sulphate when used doubtless reacting progressively with the sulphurous acid as cooking proceeds, to furnish sodium sulphite, which in turn progressively reacts upon and dissolves cementitious material in the wood. If desired, acid cooking of the wood may be. effected with spent liquors resulting from the production of sulphite pulp, particularly when such liquors contain a substantial proportion of unspent chemical, as, for instance, when wood is cooked in sodium base sulphite liquors having a high percentage of combined S0 Such spent sulphite liquors may be fortified with fresh chemicals such as sodium sulphite and/or sulphur dioxide before being used. The alkaline liquor in which fiber liberation is effected may be a solution of sodium sulphide and/or caustic soda. The inorganic content of the neutral or slightly alkaline mixture of spent liquors may be recovered and smelted in a suitable furnace or retort, either under a reducing or oxidizing atmosphere, a reducing atmosphere preferably being employed, as it facilitates subsequent steps and makes possible a saving in lime. lVhen the smelting furnace is operated under reducing conditions, the smelt consists essentially of a mix ture of sodium carbonate and sodium sulphide. The sodium carbonate may be separated from the sodium sulphide as by crystallization and the separated sodium carbonate treated with sulphur dioxide to produce a sulphurous acid solution of sodium bisulphite, suitable for cooking other raw wood, while the rest of the smelt, consisting largely of sodium sulphide, may be used for converting the acid-cooked wood into pulp. Of course a certain amount of sodium carbonate maybe permitted to remain along with the sodium sulphide, in which case this sodium carbonate may be causticized so that the liquor employed for cooking the acid-cooked wood will consist of a mixture of sodium sulphide and sodium hydroxide. In practice, only that amount of sodium carbonate need be separated from the smelt by crystallization to furnish base material for the preparation of the acid cooking liquor. In case no base is used in the acid cooking liquor (e. g., when the cooking liquor consists of sulphurous acid or a mixture of sul hurous and sulphuric acid no sodium carbonate need be crystallize out from the smelt solution and the entire sodium carbonate content of the smelt may be causticized, the resulting liquor being of about, say, 25% to 45% sulphidity and accordingly corresponding to the sulphidity of the usual kraft liquors. Losses occurring in the cycle may bemade up by adding sodium sulphate to the mixture of spent liquors before delivery into the smeltin furnace, or sodium sulphate may be adde to the acid sulphite liquor prepared from the regenerated sodium carbonate. It is thus seen that the use of lime, necessary in the usual alkaline cooking processes, may be partly or wholly dispensed with, the sodium and sulphur constituent necessary to replace losses being furnished in the orm of a comparatively inexpensive compound, namely, sodiuni sulphate. When the smelting furnace is operated under oxidizing conditions, the resulting smelt essentially consists of a mixture of sodium carbonate and sodium sulphate. A portion of the smelt may be treated with sulphur dioxide to produce an acid liquor suitable for acid sulphite cooking, while the rest of the smelt may be causticized with lime to produce a caustic soda cooking liquor. For instance, sodium carbonate may be removed as by selective crystallization from the smelt solution, then causticized and used for the preparation of an alkaline cooking liquor, while the rest of the smelt solution containing sodium sulphate may be treated with sulphur dioxide for the preparation of a fresh acid cooking liquor. Make-up chemicals may be added, as previously described, as when smelting takes place under a reducing atmosphere.

The process of the present invention will now be described as being practised with an acid cooking liquor consisting of a sulphurous acid solution of sodium bisulphite and an alkaline cooking liquor consisting essentially of a sodium sulphide .solution, the various steps of the process and the flow of the ma terials therein being illustrated on the accompanying drawing, which constitutesa flow sheet.

Referring to the drawing, 1 re resents a digester of a type used in acid coo ing, into which chipped wood such as spruce, together with an acid sulphite liquor of the desired composition, may be delivered. The liquor may vary in its composition, depending upon the conditions to be employed in cooking. For example, an acid cooking liquor having a high combined S0 content, say 4% combined and 4% free S0 may be employed,

in which case cooking may be efi'ected under atmospheric or superatmospheric pressure conditions. When using such a hquor, cooking may be carried out at a temperature of about 212 F. and under atmospheric pressure for about eight to twelve hours, but if the temperature and pressure are raised, the time of cookin may be shorter. For instance, by cooking in such a liquor of 250 to 260 and under a pressure of 30 to 35 pounds, the time may be cut down to about two to six hours, to effect the desired action upon the wood. When using an acid sulphite li uor which contains more free S0 than combmed S0 it is necessary to employ pressure in order to avoid loss of free S0 For instance, when an acid sulphite cooking liquor containing, say, 1% to 2% combined S0 and about 5% free S0 is employed, the temperature may be 250 to 260 and the pressure 60 to pounds, and cooking may be carried out for about two to six hours. At the end of cooking under any of the conditions hereinbefore outlined, the chipped wood is not changed to any marked egree physically, except that it is somewhat lighter in color and softer than the original wood, although it is impossible to shred or macerate it. to produce pulp. The spent liquor is a pale yellow color, indicating that a solution of a portion of the encrusting content of the wood has been effected. When the liquor contains a sulphuric acid, the wood assumes a reddish or brownish co or.

The cooked charge may be delivered into a washer 2, the spent liquor being washed from the wood and delivered into a storage tank 3, from which a portion may be withdrawn and reused in, the digester l for the cooking of other raw wood. The rest of the spent liquor may be subjected to recovery treatment, as will hereinafter be described. The wood delivered from thewasher 2 may be passed into a digester 4 of a type used for alkaline cooking, together with an alkaline cooking liquor of the desired composition. Various types of alkaline liquors containing sodium compounds, such as caustic soda, may be used, but it may be preferable to use one having sodium sulphide present therein, as the final pulp is of better papermaking quality when using a high sulphidity liquor than when using a straight caustic soda liquor, but when a final product of high alpha cellulose content is desired, it may be preferable to use alkaline liquorscontaining ap preciable quantities of caustic soda, which is more eflt'ective in reacting upon and dissolving less resistant celluloses. Inasmuch as a substantial portion. of the cementitious content of the wood has been removed, the cooking liquor may be of lower than usual alkalinity, but, if desired, the liquor may be of the usual strength and contain, say, about two pounds of alkali per cubic foot, calculated as Na O. The use of liquors of high alkali concentration makes possible liberation of fiber at lower temperatures, in pracv tice it being found that a cooking liquor containing two unds of alkali per cubic foot results in fi r liberation when cooking takes place at 285 F. and under 53 pounds pressure for about four hours. This time may be reduced to about three hours when cooking is effected in a liquor of similar alkali concentration at a temperature of about 335 F. and under 110 pounds pressure. After the alkaline cook has been completed, the digester charge may be delivered into a washer 5, in which the pulp is washed free from the spent cooking liquor, which may be delivered into a storage tank 6. A portion of the spent alkaline liquor may be withdrawn and used for the cooking of other acidcooked wood coming from the washer 2. The rest of the spent alkaline liquor may be mixed with the spent acid liquor accumulating the storage tank 3, the alkali content of the spent alkaline liquor serving to neutralize the acid content of the spent acid liquor. The neutral or alkaline mixture of liquors may then be passed into an evaporator 7 where a portion of its aqueous content may be removed so that economical smelting of its inorganic content may be effected. The concentrated mixture may then be delivered into a furnace 8, in which the organic ingredients are burned and its valuable inorganic content may be smelted under a reducing atmosphere to produce a smelt consisting essentially of sodium carbonate and sodium sulphide, the hot smeltbeing allowed to flow into a body of water maintained in a dissolving tank 9. Preferably, the hot solution of smelted compounds thus produced is of a concentration permitting selective crystallization of sodium carbonate when it is cooled. Crystallization of sodium carbonate may be accelerated by passing the smelt solution through a cooler 10, the crystallized sodium carbonate being removed from the mother liquorcontaining the more soluble sodium sulphide in solution. The crystals of sodium carbonate may be dissolved in water in a tank 11 and the solution thus produced used to absorb acidic gas and sublimed compounds being generated in the smelting furnace 8. As shown the sodium carbonate solution is passed downwardly through a scrubbing tower 12 into direct countercurrent contact with furnace gases delivered at the lower portion of the tower. The sodium carbonate.

solution serves to absorb sulphur dioxide present in the furnacegas, a portion of the sodium carbonate thus being converted to sodium sulphite. The liquor coming from the scrubbing tower 12 may then be treated with sulphur dioxide, as by passing it downwardlythrough a tower 13 intodirect counter current contact with sulphur burner gases being passed into the lower portion of the tower. The sulphur dioxide reacts with the sodium carbonate in solution to produce a the top of the tower. The acid sulphite liqnot thus produced may be passed into a storage tank 14, from which it may be withdrawn for use in the digester 1. In usual practice, it will not be necessary to remove all the sodium carbonate from the smelt solution, as the alkali required for the preparation of the alkaline cooking liquor is usually much greater than the alkali required for the preparation of the acid cooking liquor, and in case acid cooking is effected in an acid liquor containing no sodium salts, no removal of sodium carbonate from the smelt solution will be effected.

The mother liquor from which the sodium carbonate has been crystallized and which contains sodium sulphide together with residual uncrystallized sodium carbonate in solution, may be delivered into a tank 15 and diluted to the strength desired for alkaline cooking. 'In some cases, however, as when the mother liquor contains a substantial amount of sodium carbonate, its sodium carbonate content is preferably converted into caustic soda, in which case it may be delivered into a causticizer 16, wherein it may be treated with lime to effect causticization of its sodium carbonate content, whereupon the causticized liquor, consisting of a solution of sodium sulphide and caustic soda, may be withdrawn for use in the digester 4. Losses of sodium and sulphur constituent occurring in the cycle may be replaced as by adding sodium sulphate to the neutral or alkaline mixture of spent liquors prior to their delivery into the furnace 8, or by adding caustic soda to the fresh alkaline liquor and sulphur to the mixture of spent liquors.

Using processes such as hereinbefore described. it is possible to produce easy bleaching pulps high in alpha cellulose content and having excellent physical characteristics including high strength and unusually high tear resistance. Accordingly, such pulps are eminently suitable for use as a raw material in the manufacture of high grade bond, ledger, and writing papers, and in the preparation of cellulose derivatives and products such as artificial silk films, parclnnents, and

Spruce chips may be cooked for about four 1 hours at 250 F. and under 60 pounds pressure in an acid sulphite liquor containing 4% free and 4% combined S0 whereupon the cooked chips, after washing if desired, may then be cooked at 335 F. and under 110 sulphidity.

pounds pressure for about four hours in a 1i 1101' containing one pound of alkali per cu ic foot, calculated as Na O, and of 40% Example 2 Spruce chips may be cooked for about five to six hours at 225 F. and under a maximum pressure of about 75 pounds, in a sulphurous acid solution containing about 5% S0 The cooked chips, after being washed free of liquor, may then be cooked at 335 F. and under 110 pounds pressure for about four hours in an alkaline liquor containing about 1 to 2 pounds of alkali per cubic foot, calculated as No. 0 and of sulphidity of, say, from 0% to 50%.

I present herein a table giving various physical and chemical characteristics of a usual unbleached sulphite pulp, a usual unbleached kraft pulp,'and representative unbleached pulps produced as described in the foregoing examples:

Unbleached sulphiie pulp It is thus seen that pulps produced as herein described diiier greatly from both sulphite pulp and kraft pulp in significant physical and chemical characteristics. The usual kraft pulp has lower tear resistance and is of lower alpha cellulose and much higher pentosan content. Sulphite pulp is much inferior in its physical characteristics, including strength and tear resistance, and has a distinctly lower alpha cellulose content. Pulp produced as herein described may be readily bleached with comparatively low bleach usage into a product of high whiteness eminently suitable for use as a cotton fiber substitute in the manufacture of cellulose products. When raw ccllulosic material is cooked in acid liquors containing only sulphurous acid or a mixture of sulphurous and sulphuric acids, and then cooked in alkaline liquors as hereinbefore described, the product is unusually low in pentosans, as indicated in the foregoing table, as acid liquors of this type are unusually reactive upon pentosans, the high acid content evidently effecting a substantially complete hydrolysis and solution of the pentosans from the raw material. In an acid sulphite liquor, the lower the combined S0 content, the easier it appears to produce a pulp of low pentosan content.

If desired, the pulp may be refined before it is subjected to bleaching to remove bleach-consuming constituents such as ligneous and colorlng matter. For Instance, when the pulp contains a substantial amount of ligneous matter as a result of undercook-- ing in the acid liquor or cooking in an acid liquor of low chemical strength, it may be advantageous to chlorinate the pulp resulting from alkaline cookin with, say 1% to 2% chlorine, based on the dry weight of pulp, and then to cook the pulp again in an alkaline liquor. Final bleaching of the pulp to pure whiteness may then be accomplished with a minimum bleach usage and without degrading the alpha cellulose content of the pulp. Such a refining procedure may be advantageous even when the pulp produced by successive acid and alkaline cooks has an alpha cellulose content of 95% to 96% and a pentosan content of 1%, as such pulp may require so much bleach for whitening as to undergo degradation. Preferably, the bleach liquor is maintained distinctly alkaline during the entire bleaching operation, in order to minimize degradation of the fiber.

One of the important advantages of a two-step cooking process such as described is that it makes possible the use of a raw cellulosic material such as wood in the form of chips or other units of much larger size than the chips ordinarily employed in the production of chemical wood pulp. The procedures hereinbefore given have been successfully performed upon l-inch cubes of wood as the raw material, and the resulting pulps had unusually high strength and tear resistance and excellent. chemical characteristics, as already described. After chips or cubes have been cooked in acid liquors, as hereinbefore described, they may advantageously be exposed to the action of chlorine gas or chlorine water to effect reaction upon and removal of ligneous substance or torender such substance more susceptible to solution in alkaline liquors, so that subsequent fiber liberation in alkaline liquors and final bleaching may be facilitated. For example, the acid-cooked chips may be steeped in chlorine water containing, say, about 6% chlorine, based on the weight of ry chips, for about six hours at room temperature, and then, after washing, cooked in alkaline liquors as hereinbefore described.

The process of the present invention may be practised advantageously upon raw cellulosic material other than wood or upon wood reduced to smaller size than chips, e. g., wood in ground condition, such as ground wood or mechanical wood pulp, or in comminuted condition, such as sawdust. When bagasse, bamboo or like-raw material which is comparatively high in pentosans is employed,'the resulting pulp will have a much lower pentosan content than pulps produced from such raw material by the usual cooking processes. Inasmuch as it is dilficult to pulp bamboo or tards pulping. with an acid liquor, the usual practice is to use alkaline liquors for pulping such material, such liquors being only mildly reactive upon pentosans. According to the process of the present invention, however, such raw material is first cooked in acid liquors, which are highly reactive upon pentosans without completing fiber liberation, whereupon the acid-cooked material is then cooked in an alkaline liquor to complete fiber liberation. The acid liquor may be strongly acid and consist of a sulphurous acid solution alone containing, say, 5% free S0 or it may additionally contain about 2% salt cake (Na- 30,), or niter cake (NaHsO or about 0.5% sulphuric acid.

When ground wood or sawdust is used as a raw material, the final pulp will have chemical characteristics similar to the product resulting from chipped wood, but its physical or papermaking characteristics will be quite different owing to the short fiber length of the raw material. Such a pulp will be highly suitable for conversion into cellulose Xanthate or other cellulose derivatives, since being short-fibered, it is highly reactive toward converting chemicals. When acid cooking is effected in a strongly acid solution, such a short-fibered product is low in pentosans, and accordingly eminently suitable as a raw material for cellulose derivative preparation. While a purified short-fibered pulp of this type does not alone yield papers of good physical characteristics, nevertheless, because of its stability and purity, it may be used to advantage as a filler or bulk material along with pulps of higher fiber lengths, serving to impart opacity, resistance to curling, and other desirable characteristics to the finished paper. When using ground wood as a raw material, it is possible to produce a final pulp of a given purity with milder cooking liquors than those necessary for chipped wood. The ground wood may be cooked as a 6% suspension in an acid liquor containing 4% free S0 and 4% combined S0 at a temperature of 250 F. for about four hours. The cooked material may then be washed free of acid liquor and cooked in an alkaline liquor containing from 0.5 to 1 pound active Na O per cubic foot. and of 40% sulphidity, at a temperature of 300 to 330 F., for about two hours. The final product may be bleached to high whiteness with from 3% to 5% bleach, based on the weight of dry fiber. If it is desired, to produce a final product which is Very low in pentosans, say, less than 1%, acid cooking mav be accomplished in a liquor containing free S0 alone, or in an acid liquor containing free S0 and sodium salts such as salt cake or niter cake, or sulphuric acid. If desired both the acid and alkaline cook may be carried out under atmospheric pressure conditions, but the higher temperatures at tainable under super-atmospheric ressures make possible a much shorter perio of processlng.

Having thus described the process of the present invention and various commercial applications thereof, it should be evident to those skilled in the art that changes or modifications might be resorted to without departing from the spirit or scope of invention as de ned in the appended claims. Owing to the lack of a better generic or comprehensive term, by the expression raw cellulosic material as used in the appended claims, I mean various raw materials of the type herein described including wood in variousphysical conditions, for instance, in the form of cubes,.chips, ground wood, or sawdust.

I claim:

1. A process which comprises cooking raw cellulosic material in a sulphurous acid solution containing positively added and free sulphuric acid short of effecting fiber liberation into a pulp, and then cooking in an alkaline cooking liquor to,complete fiber liberation.

2. A process which comprises cooking raw cellulosic material in an acid liquor short of effecting fiber liberation into a pulp, treating with chlorine, and then cooking in an alkaline liquor to complete fiber liberation.

3. A process which comprises cooking raw cellulosic material at a temperature of at least about 212 F. in a sulphurous acid solution of sodium-sulphur salts short of effecting fiber liberation into a pulp, whereby chemical injury to isolated pulp fibers is avoided, and then cooking in a substantially fresh alkaline liquor containing sodium sulphide to complete fiberliberation.

4. A process which comprises cooking raw cellulosic material in a sulphurous acid solution of sodium sulphite and sodium sulphate short of effecting fiberliberation into a pulp, and then cooking in an alkaline liquor to complete fiberliberation.

V 5. A process which comprises cooking raw cellulosic material at a temperature of at least about 212 F. in a sulphurous acid solution of sodium sulphite short of effecting fiber liberation into a pulp, whereby chemical injury to isolated pulp fibers is avoided, and then cooking in a substantially fresh alkaline liquor containing sodium sulphide to complete fiber liberation;

6. A process which comprises cooking raw cellulosic material in a sulphurous acid solution of sodium-sulphur salts, short of effecting fiber liberation, cooking in an alkaline solution of sodium compounds to complete fiber liberation, mixing spent liquor resulting from both cooking operations, smelting the inorganic content of the mixture in a reducing atmosphere to produce sodium carbonate and sodium sulphide, separating practically only sodium carbonate from the smelt, treating the separated sodium carbonate with sulphur dioxide to produce an acid liquor for subsequent first-mentioned cooking, and using the rest of the smelt for subsequent second-mentioned cooking.

7. A process which comprises cooking raw cellulosic material in a sulphurous acid solution of sodium-sulphur salts short of efl'ecting fiber liberation, cooking in an alkaline solution of sodium compounds to complete fiber liberation, mixing spent liquor resulting from both cooking operations, smelting the inorganic content of the mixture in a reducing atmosphere to produce sodium carbonate and sodium sulphide, separating practically only sodium carbonate from the smelt, treating the separated sodium carbonate with sulphur dioxide to produce an acid liquor for subsequent first-mentioned cooking, causticizing the rest of the smelt, and using it for subsequent second-n'ientioned cookin 8. A cyclic process which comprises cooking raw cellulosic material in a sulphurous acid solution of sodium sulphite short of etfecting fiber liberation, cooking in an alkaline solution containing sodium sulphide to complete fiber liberation, mixing spent liquors resulting from both cooking operations, smelting the inorganic content of the mixture in a reducing atmosphere to produce sodium carbonate and sodium sulphide, dissolving the smelt in Water to produce an aqueous smelt solution, selectively crystallizing practically only sodium carbonate from the solution,

treating the sodium carbonate with water and sulphur dioxide to form a solution for subsequent first-mentioned cooking, and using the mother liquor for subsequent second-mentioned cooking.

9. A cyclic process which comprises cooking raw cellulosic material in a sulphurous acid solution of sodium-sulphur salts short of eflecting fiber liberation, cooking in an alkaline solution of sodium compounds to complete fiber liberation, mixing spent liquors resulting from both cooking operations, smelting the inorganic content of the mixture in an oxidizing atmosphere to produce sodium carbonate and sodium sulphate, dissolving the smelted compounds in water to produce a smelt solution, removing practically only sodium carbonate from the smelt solution, cauticizing such removed sodium carbonate for subsequent second-mentioned cooking, and treating the rest of the smelt solution with sulphur dioxide to produce an acid liquor for subsequent second-mentioned cookmg.

10. A process which comprises cooking raw cellulosic material of the character of chipped wood in a solution containing nit-er cake to'react upon and dissolve more especially the pentosans present in such material while maintaining the fibers bound together, washing such material, and then cooking the washed material in an alkaline liquor to iso late the ultimate fibers.

In testimony whereof I have affixed my signature.

GEORGE A. RICHTER. 

